This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Zeolites are a class of crystalline, microporous, silica-based molecular sieves of varying topology (micropore size and interconnectivity). They are pure-silica (SiO2) materials containing some fraction of their Si atoms substituted with Al, which generate ion-exchange sites and catalytic sites. Zeolites are used in adsorption, ion-exchange and catalytic applications, and especially in the chemical and petrochemical industry as acid catalysts. Current synthesis routes only are specified to prepare a specific crystal structure and bulk elemental composition (Si/Al ratio).
Brønsted acidic zeolites are silica-based molecular sieve frameworks that contain a fraction of their Si atoms substituted with Al atoms, which generate anionic charges balanced by protons that differ in intrazeolite location among known crystal topologies and in density with changes in bulk composition (Si/Al ratio). Yet, even a given zeolite at fixed composition contains catalytic diversity conferred by differences in the arrangement and distribution of its framework Al atoms, because reactive intermediates and transition states formed at their attendant Brønsted acid sites are also stabilized by van der Waals interactions with surrounding oxide cavities. One type of Al arrangement describes the location of Al atoms among different pores of a given zeolite, as in the case of ferrierite (FER) zeolites (Si/Al=10-20) that contain higher fractions of Al within 8-membered rings (8-MR) when crystallized with smaller pyrrolidine (˜0.46 nm kinetic diameter) organic structure-directing agents (SDAs) than with larger SDAs (e.g., benzylmethylpyrrolidinium, hexamethyleneimine), and that catalyze dimethyl ether carbonylation to methyl acetate with higher turnover rates (per total Al; 473 K) because this reaction occurs with high specificity within 8-MR voids that solvate carbonylation transition states more effectively than 10-MR and larger voids. Another type of Al arrangement describes the proximity of Al atoms within the framework, ranging from the limit of Al site isolation (Al—O(—Si—O)x—Al, x≥3 to higher densities of proximal or “paired” Al atoms (Al—O(—Si—O)x—Al, x=1, 2), which has been recognized, but not controlled predictably during zeolite synthesis. For purposes of this disclosure, is structure-directing agent is a compound that is present during crystallization of the zeolite and helps guide the formation of the desired crystal structure.
The Al distribution in zeolites has been linked to structural stability, deactivation and coking in acid catalysis with hydrocarbons and alcohols, so manipulating this distribution can benefit those technologies. Aluminum distribution in zeolites has also been linked to the numbers and structures of extraframework metal ions (e.g., Cu2+, (CuOH)+) that can be exchanged onto the zeolite, and because these ions are catalytic sites in NOx (x=1,2) selective catalytic reduction with ammonia, manipulating this distribution can benefit those technologies.
The problem of how to control Al distribution in zeolites is currently being addressed by changing various zeolite synthesis variables, including the Si source, Al source, Na source, counteranion (OH, Cl, PO4, NO3), the organic and inorganic additives used, etc. These changes have not been systematically made, and have been studied for other zeolite structures including MFI (or ZSM-5), and have not led to systematic changes in the Al distribution.
An International Patent application, No. PCT/CZ2010/000113 by Oleg Bortnovsky et. al, titled “Method of manufacture of zeolites with pentasil structure with controlled distribution of aluminium atoms in the skeleton” (Publication Number WO2011095140 A1), whose contents are incorporated herein by reference in their entirety into this disclosure, a method of manufacture of microporous zeolites with pentasil structure with controlled distribution of aluminium atoms in an aluminosilicate tetrahedrally coordinated skeleton in “Al pairs” in (Al-O—(Si—O)n=1,2-Al) sequences localized in a single ring and in different rings in Al-O—(Si—O)n>2-Al sequences.
Chabazite (CHA) zeolites do not have a pentasil structure (i.e., composed of 5-membered rings), and belong to a different class of zeolites composed of 6-membered ring building units. While the previously reported methods, which change the relative amounts and ratios of chemical ingredients to synthesize a zeolite, may be applied to chabazite zeolites, it is unclear exactly how these changes would influence the Al distribution. Pentasil zeolites also have more than 1 crystallographically-unique tetrahedral site (T-site) in the lattice, while CHA zeolites only have 1 crystallographically-unique T-site in the lattice. Therefore, since the underlying mechanisms controlling Al distributions in pentasil zeolites are unknown, the specific strategies used to control Al distribution in pentasil zeolites would not apply to CHA zeolites, because they only contain 1 possible lattice T-site location for Al substitution.
Hence, there is unmet need for synthetic procedures that directly and systematically control the Al distribution in chabazite zeolites at a fixed Si/Al ratio, by only manipulating the type and amount of structure-directing agents used. Further, it is desirable to change the amounts and types of organic and inorganic cations used as structure-directing agents, which leads to clear and systematic changes in the Al distribution in CHA (SSZ-13). Meeting these needs will benefit structural stability, deactivation and coking in acid catalysis with hydrocarbons and alcohols, and will benefit the structural stability and catalytic rates of redox catalysis that occurs on metal ions exchanged onto acid zeolites, such as Cu or Fe ions exchanged onto CHA zeolites for NOx selective catalytic reduction (SCR) with ammonia.